The present invention relates to a hydraulic control circuit including a dual pressure relief valve which is operatively responsive to operating conditions of a hydraulic motor for providing actuating fluid to the motor at two pressure levels. More particularly, the circuit is employed to operate a pushpull coupling adapted for tandem operation of two vehicles.
A push-pull coupling of the type contemplated by the present invention is described in substantial detail within U.S. Pat. No. 3,434,738, issued Mar. 25, 1969, and assigned to the assignee of the present invention. Such push-pull couplings include a hook member on one vehicle and a latch member on another vehicle to permit rapid coupling and uncoupling for example of separate tractor-scraper units. Accordingly, two such scrapers may operate in tandem to employ the power of both vehicles during loading. After the scrapers are loaded, they may be rapidly uncoupled and operate independently during transport and unloading.
The latch member is preferably positioned by a double-acting hydraulic jack which operates in a first direction to urge the latch member into engagement with a hook on the other vehicle or in the opposite direction to retract the latch member. In common control circuit for such a jack, a pump or source of fluid continues to direct fluid under pressure toward the jack even when the latch means is in its retracted position. However, since substantially reduced fluid pressure is required to maintain the latch member in its raised position, it is desirable to provide means for lowering the operating pressure in order to avoid unnecessary operation of the pump against a relatively higher operating pressure.
Within the prior art, this problem has been partially solved by means of by-pass devices providing a relatively low operating pressure, for example 200 psi, when the vehicle engine is running at low idle and approximately 500 psi, for example, when the engine is operating at high idle. Such an arrangement, however, does not necessarily conform the relative operating pressure with operating requirements of the jack.
Another problem encountered within such couplings is that the jack may also be employed as a stop for positioning the latch member in a retracted position. Accordingly, any inadvertent movement of the latch member by external forces can result in severe damage to the jack or its mounting. One prior art solution to this particular problem has been the use of a mechanical stop secured to the vehicle frame in a position to abut the latch member when it is fully retracted. This solution creates another problem however, in that the retracted position of the latch member may vary from vehicle to vehicle or within a single vehicle during continued operation. Thus, operation of the jack for opening the by-pass device may not always correspond with the fully retracted position of the latch member. Accordingly, there remains a possibility that the pump will continue to work against the relatively higher pressure in the control circuit.
Additional control circuits or components employable within prior art circuits of the type discussed above include U.S. Pat. Nos. 3,469,861; 3,160,076; 3,416,561 and 2,905,190.